1. Field of the Invention
The present invention relates to red luminous ink compositions containing a europium complex the absorption wavelength of which is within the ultraviolet region and the fluorescent luminescence region of which is within the red region.
2. Description of the Related Art
In recent years, the term “functional organic compound” has come to be used and organic substances for use in electronic or optical devices are actively studied. Of such substances, luminous compounds (self-luminous compounds) capable of showing a photoluminescence (PL) phenomenon are known.
One application of luminous compounds is security ink. The security ink is an ink such that a handwriting is invisible under visible light, but when being irradiated with ultraviolet rays, e.g., a black light lamp, the handwriting emits light and one can read the information recorded.
The security ink is used for the purpose of preventing forgery or reproduction or of recording secret information. For instance, if lot numbers or codes of items are recorded with a security ink, it becomes easy to trace the distribution channels of the items or to prevent production or distribution of counterfeit products, and the fear of manipulation or damage of recorded information is reduced because the information is invisible under visible light.
In Japanese Patent Laid-Open Publication No. 2000-144029, proposed is an ink composition with the following features: being substantially invisible within the visible light region; containing europium (Eu) complex having an emission center wavelength at 615±20 μm when being excited with ultraviolet rays; containing a polyvinyl resin; containing, as a neutral ligand, at least a phosphoric acid type compound selected from a phosphine oxide compound, a phosphine sulfide compound and a phosphine compound; and containing not less than 94% of water and/or ethanol in a solvent.
In Japanese Patent Laid-Open Publication No. 2000-160083, proposed is an ink composition for ink jet printing containing, as a luminous material, from 0.001 to 5% by weight of 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate.europium chelate compound and also containing not less than 70% by weight of an alcoholic solvent as a solvent.
With respect to the ink composition using a europium complex as a luminous compound, known are an aqueous ink that becomes visible when being irradiated with ultraviolet rays (Japanese Patent Publication No. S54(1979)-22336), a fluorescent europium complex for thermal transfer (Japanese Patent Publication No. H6(1994)-15269), a luminous ink composition containing a europium complex in which a bidentate ligand, e.g., bipyridine derivative or phenanthroline derivative, coordinates to a europium (Japanese Patent Laid-Open Publication No. H3(1991)-50291), and an ink composition which contains a tetra(benzoyl trifluoroacetonate) europium complex having an ammonium salt as a counter ion (Japanese Patent Laid-Open Publications No. S64(1989)-6085 and No. S64(1989)-26583).
Considering practical use as a security ink, a handwriting or security information recorded with the ink is required to emit to a degree such that the ink is clearly visible when being irradiated with ultraviolet rays, e.g., a black light lamp (365 nm light). However, the conventional luminous compounds or ink compositions is unsatisfactory in emission intensity or have a problem in stability when the ink contains alcohol as a solvent.
There is therefore a demand for providing a red luminous ink composition that exhibits excellent emission intensity such that a handwriting or image recorded which is substantially invisible within the visible light region can be visually recognized clearly.